The attendant was badly shaken up after the incident and received help from Victim Support.
Mr Davis said when he reviewed CCTV footage of the incident, he was impressed by the way the middle-aged Dannevirke man handled the situation.
"The Caltex worker did the right thing and opened the till. The offender then helped himself to the cash by leaning over the counter. I'm pretty impressed with what [the attendant] has done. He was cool, calm, and he did everything right, following instructions and not irritating the offender in any way."
The alleged offender was wearing a hoodie, with material covering his face. He was also wearing Red Band gumboots and a pair of Canterbury track pants.
Police believed the offender was likely to be aged under 30, because of his build.
"To me he looks reasonably athletic, but not solid. Looking at the security camera footage, he doesn't look overly tall, probably somewhere between 5 foot 8 inches (1.7m) and 5 foot 11 inches (1.8m)."
Police were asking the public for any information in relation to the incident.
"The offender has left the service station on foot and there was some passing traffic so someone may have seen something."
Caltex Westlow owner Maitlinn Williams declined to comment on the robbery, but said there had been "various incidents" during his time running the gas station.
-Dannevirke Police want your help in tracking down the offender. You can phone Senior Sergeant Nathan Davis on 06 374 4503, or Crimestoppers on 0800 555 111. All information will be treated confidentially.